This invention relates to an electric shutter operating circuit .[.of.]. .Iadd.for .Iaddend.photographic cameras, more particularly, single lens reflecting cameras.
A shutter operating circuit is used for the purpose of setting a shutter time in accordance with the brightness of an object to be photographed and in a prior art shutter operating circuit a photoelectric element or transducer has been used in most cases as a light receiving element. However, in an shutter operating circuit utilizing a photoelectric element as the light receiving element it is extremely difficult to maintain at a definite value the rate of varying of the resistance value of the element for varying brightness of the object, that is the .gamma. value for the .[.brigtness.]. .Iadd.brightness .Iaddend.varying over a wide range. In other words, the contrast of the picture or the quantity of light exposure of the photographic film varies with the brightness of the object. Furthermore, the photoelectric element varies its resistance with a time lag in response to the variation in the quantity of received light so that when the brightness of the object varies rapidly it is difficult to assure proper exposure due to said time lag. For this reason, it has been proposed to obviate this difficulty by providing in front of the photoelectric element an iris diaphragm of complicated shape which varies its opening in accordance with the iris setting or the sensitivity of the photographic film.
When such an iris mechanism is mounted in front of the photoelectric element it is possible to compensate for the .gamma. value but it is necessary to mechanically couple such iris mechanism with an ordinary iris mechanism for a lens or with a ring giving an information regarding the film sensitivity. Accordingly, the mechanism becomes extremely .[.complicate.]. .Iadd.complicated .Iaddend.and expensive. Furthermore, as the response characteristic of the photoelectric element is poor, when a picture is photographed by using a device which .[.complete.]. .Iadd.completes .Iaddend.flashing in a short time, such as a stroboscope, it is impossible to set a proper shutter time by taking into consideration such flashing .[.of.]. .Iadd.for a .Iaddend.short time. Moreover, with the arrangement described above, the quantity of light impinging upon the photoelectric element is reduced in accordance with the opening of the iris diaphragm associated with the lens, the response characteristic of the photoelectric element in dark conditions becomes poor.